When native starch granules are dispersed in water and heated the granules hydrate and swell at a temperature of 60.degree. C. The starch suspension shows a peak viscosity at temperatures between 65 and 95.degree. C. The increased viscosity is a desirable characteristic in many food applications. The swollen granules are however very fragile and tend to disintegrate which leads to a decrease in viscosity. Shear or extreme pH conditions tend to speed up this disintegration process.
High viscosity starches are starches which show an important increase in viscosity characteristics upon heating. Starches with a high swelling power are those derived from waxy varieties and starches derived from tubers and roots (e.g. potato, tapioca). The swollen granules burst to a large extent during heating which leads again to a drastic viscosity breakdown.
In order to overcome the undesirable viscosity breakdown starches may be stabilized. The viscosity breakdown can be overcome by treating the starch granules with chemical reagents. These reagents introduce intermolecular bridges or cross-links between the starch molecules. Stabilized high viscosity starches show substantial swelling of the granules on heating without disintegration of the swollen granules on prolonged heating. The integrity of the highly swollen starch granules in the heat stage guarantees a stable paste viscosity.
Typically cross-bonding of starch is performed with reagents such as phosphorus oxychloride, sodium trimetaphosphate, adipic anhydride, epichlorohydrin etc. In order to obtain stabilization at the highest possible viscosity level very low amounts of cross-linking agent are applied. The characteristics of the final product are very much dependent on the reaction conditions which have been applied. Small deviations in the dosed amount and slight changes in reaction time give rise to modified starches with an unpredictable viscosity behavior.
Highly swollen (viscous) cooking stable starches are used in many different applications, for example in the preparation of soups, sauces, meat products, dressings, micro-wavable food and in the preparation of bakery creams and fillings. Convenience foods need to have a high viscosity and smooth texture after heating (to 80-100.degree. C.). Stabilized high viscosity starches are particularly suitable for the mentioned applications.
The field of applications further includes instant thickening food preparations when these starches are used in a pre-gelatinized form obtained by e.g. by roll-drying.
Chemical modification of starches is a well-known process the process has been described in several patents. With respect to background of the present invention the following patents may be relevant.
JP-B-07106377 (Japan Maize Products, Derwent publication Apr. 2, 1971, page 6 AN 11 12945S) describes the oxidation of starch. This patent describes a strong oxidation as it is mentioned that the temperature of the start of the gelatinization descends. It can be concluded that the conditions are such that the starch is de-polymerized.
U.S. Pat. 1,937,543 describes a method of making starch. It is described that an undesirable excess of sulfur dioxide is removed by the addition of a determined amount of sodium hypochlorite sufficient to oxidize the sulfur dioxide. Only a slight excess of the hypochlorite is used as an excess of chlorine may detrimentally affect the taste of the finished product. Starch is thus reacted with a slight excess of hypochlorite in relation to the sulfur dioxide which is present in an amount of from 15 to 20 parts per million. The product is reported to become thicker boiling.
U.S. Pat. No. 2,108,862 describes a process of making halogenated thick boiling starches. The reaction is performed at acidic pH.
U.S. Pat. No. 2,317,752 describes a method of producing what the inventors call `inhibited` starch i.e. starch differing from raw starch in that it yields upon cooking in aqueous media a `short` smooth and stable paste. It is reported that it is known that the reaction of starch with hypochlorite gives thin-boiling starches. Inhibited starches according to this patent are obtained when hypochlorite is used in combination with a considerable amount of a reaction modifier.
U.S. Pat. No. 2,354,838 describes a method for thinning waxy maize starch by hypochlorite treatment. The products obtained according to this patent are low viscosity starches with excellent clarity, adhesive strength and no retrogradation tendency.
U.S. Pat. No. 2,989,521 describes a method of cross-linking and oxidizing starch. The starting material for the hypochlorite treatment is extremely highly cross-bonded starch obtained with epichlorohydrin and with no measurable viscosity. This starch is then treated with high amounts of hypochlorite in order to weaken the structure and increase the viscosity. The diglycerol ether bonds are not broken during the alkaline hypochlorite treatment. By this treatment a large number of carboxyl groups (3-9 mol/100 (AGU) are introduced. Due to these hydrophylic groups the starch can better swell in water and develops a high viscosity.
U.S. Pat. No. 4,281,111 describes a hypochlorite treatment of starch at pH of around 3 followed by a hydroxypropylation.
Extensive chemical cross-bonding of starches is not allowed when the products are to be used in food applications. There is therefore a need for starch products which have been treated under mild conditions and which nevertheless have acquired the characteristics of a heat stable high viscosity starch. Moreover such mildly treated starches are allowed as ingredients in food products.